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  1. #1
    wilsonneal's Avatar
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    sodium dichromate question

    Quick question (which reveals I didn't pay any attention in chemistry class, I'm sure):

    The notes I've read on using dichromate in the PotOx to control contrast specify "Sodium Dichromate". When I shop for this, however, I see "Sodium Dichromate Dihydrate". Are these the same thing? In Arentz he gives the formula as NA2 CR7 O7, and I've seen the Dihydrate adds 2H20 to the formula. So, are they interchangeable, given that I am just adding the powder to distilled water?

    And, is potassium dichromate interchangeable?

    Thanks,
    Neal

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    Quote Originally Posted by wilsonneal View Post
    Quick question (which reveals I didn't pay any attention in chemistry class, I'm sure):

    The notes I've read on using dichromate in the PotOx to control contrast specify "Sodium Dichromate". When I shop for this, however, I see "Sodium Dichromate Dihydrate". Are these the same thing? In Arentz he gives the formula as NA2 CR7 O7, and I've seen the Dihydrate adds 2H20 to the formula. So, are they interchangeable, given that I am just adding the powder to distilled water?

    And, is potassium dichromate interchangeable?

    Thanks,
    Neal
    Ammonium, potassium and sodium dichromate are all interchangeable for controlling contrast in Pt./Pd., and also kallitype. But you do not have to use the very concentrated solution recommended by Dick Arentz. A 5% solution of either will give the same contrast control, though you will have to calibrate for the level of contrast control needed. There is no purpose served in my opinion in using a more concentrated solution.

    Sandy

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    wilsonneal's Avatar
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    Thanks as always, Sandy. So to clarify: is the dihydrate version the same?
    Thanks
    N

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    Quote Originally Posted by wilsonneal View Post
    Thanks as always, Sandy. So to clarify: is the dihydrate version the same?
    Thanks
    N
    Same chemical but the different forms would require different amounts of water to have solutions of comparable strength.

    No matter since you will have to calibrate anyway.

    Sandy

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    Neal,

    Call up B&S and have Kevin mix up a bottle of 50% sodium solution.

    I think I had a 100ml bottle made, and it has lasted me several years so far. Works like a charm, and then you will be able to use Dick's drop approach in his book.


    ---Michael
    www.mutmansky.com
    B&W photography in Silver, Palladium, and gum bichromate.

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    The dichromate system of contrast control makes a lot of sense for people who are working with digital negatives that are close in DR since under normal circumstances one or two different bottles of developer should be all they would need. It makes less sense for persons printing negatives that are all over the board in terms of DR because you would need a separate bottle for every difference in DR of log 0.15 or so.

    You can control contrast with any of the three dichromates, ammonium, potassium or sodium. When used at the same strength solution there is no practical advantage to one over the other. Sodium dichromate is much more soluble than ammonium, and in turn ammonium dichromate is much more soluble than potassium, but when you put an equivalent amount by weight into the developer they all work more or less the same. I have personally avoided the use of sodium dichromate in my work in carbon printing because the powder is very delisquecent, which I find to be a considerable disadvantgage.

    So in practice if you wanted to use Dick Arentz' data (See Chart 7.6 on page 60 in the 2nd edition of his book), but have on hand ammonium or potassium dichromate and not sodium, you will get approximately the same contrast control if you just substitute by weight. For example, if you have a 25% solution of ammonium dichromate you would just use twice the number of drops recommended by Arentz for the 50% sodium dichromate solution. Or you could use a 5% potassium dichromate solution, substituting ten drops to one for the 50% sodium dichromate solution. The results may not be *exactly* the same because the equivalent weight of the three dichromates is not equal, and the percent solution you uses changes somewhat the strength of the developer, but it will be very close. And in any event the careful worker will want to test to calibrate to his/her materials.

    If you decide to mix your own percent solutions be sure to use distilled water. Mixed in distilled water dichromate solutions have very long shelf life. I have some solutions on hand that were mixed five or more years ago and they still work as well as the day they were mixed.

    Sandy King

  7. #7

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    Well sure, but I see no reason to monkey around with alternate solutions when the precise one is available for relatively little money, and it involves no hazmat charges, and no handling of dichromate powders. Once in solution, it is stable and will pose no problems for measurement or storage, as you indicate.

    Additionally, you need to adjust your formulas for the molar weight of dichromate in solution, which will impact the results a bit, so it's not necessarily as straightforward as it seems.

    For someone who has a bunch of dichromate lying around, making an alternate method makes sense, but most pt/pd printers have no reason to have dichromate powder, so purchasing a 100ml bottle of sodium dichromate makes perfect sense.

    I also see no reason to fret about having multiple bottles of developer as ong as you are mixing the developer from scratch. It keeps forever, and ultimately takes up very little room in most cases. In the end, if you are building your negatives n a consistant manner, you will use only about three different mixes most of the time anyway.


    ---Michael
    www.mutmansky.com
    B&W photography in Silver, Palladium, and gum bichromate.

  8. #8

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    Quote Originally Posted by Michael Mutmansky View Post
    Well sure, but I see no reason to monkey around with alternate solutions when the precise one is available for relatively little money, and it involves no hazmat charges, and no handling of dichromate powders. Once in solution, it is stable and will pose no problems for measurement or storage, as you indicate.

    Additionally, you need to adjust your formulas for the molar weight of dichromate in solution, which will impact the results a bit, so it's not necessarily as straightforward as it seems.

    For someone who has a bunch of dichromate lying around, making an alternate method makes sense, but most pt/pd printers have no reason to have dichromate powder, so purchasing a 100ml bottle of sodium dichromate makes perfect sense.

    I also see no reason to fret about having multiple bottles of developer as ong as you are mixing the developer from scratch. It keeps forever, and ultimately takes up very little room in most cases. In the end, if you are building your negatives n a consistant manner, you will use only about three different mixes most of the time anyway.


    ---Michael
    My purpose is not to encourage or discourage persons from buying the dichromate in solution, merely to point out that there is no functional difference in the way the three dichromates work. I don't think this point is obvious to everyone. In fact, my recollection is that a previous version of Arentz' book stated that only sodium dichromate would work. So I think the information could be potentially useful to persons who may have ammonium or potassium dichromate on hand, but not sodium. Or for those who simply like to have complete control of their chemistry.


    Sandy King
    Last edited by sanking; 03-22-2007 at 03:31 PM. Click to view previous post history. Reason: Spelling.



 

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